Technical Field
The present invention relates to an acoustic transducer that converts a sound wave into electrical signals, and to a microphone including the acoustic transducer. In particular, the present invention relates to a micro-sized acoustic transducer, which is fabricated by using a MEMS (Micro Electro Mechanical System) technology, and the like.
Description of the Related Art
Conventionally, an ECM (Electret Condenser Microphone) has been widely used as a miniature microphone mounted on a cellular (mobile) phone and the like. However, the ECM is weak against heat, and a MEMS microphone is superior to the ECM in terms of digitalization, miniaturization, enhancement of functionality/multi-functionality, and power saving. Accordingly, at present, the MEMS microphone is becoming widespread.
The MEMS microphone includes a capacitor-type acoustic sensor (acoustic transducer) that detects a sound wave and converts the detected sound wave into an electrical signal (detection signal), a drive circuit that applies a voltage to the acoustic sensor, and a signal processing circuit that performs signal processing such as amplification on the detection signal from the acoustic sensor and outputs the processed detection signal to outside. The acoustic sensor is manufactured by using MEMS technology. The drive circuit and the signal processing circuit are manufactured integrally with each other as an ASIC (Application Specific Integrated Circuit) by using a semiconductor manufacturing technology.
Recently, the microphone has been required to detect and output a large sound with high quality. In general, a maximum input sound pressure (dynamic range) is restricted by a total harmonic distortion (hereinafter, referred to as “THD”). This is because attempting to detect a large sound by the microphone results in generation of a harmonic distortion in an output signal, thereby leading to deterioration of sound quality. Namely, if the THD can be reduced, then the maximum input sound pressure can be increased.
However, in a general microphone, detection sensitivity for the sound wave and the THD have a trade-off relationship therebetween. Therefore, a high-sensitivity microphone has a large THD, so as to have a small maximum input sound pressure. This is because the high-sensitivity microphone tends to output a large signal and therefore is likely to cause the THD. Meanwhile, a low-sensitivity microphone causes a small THD, so as to have a large maximum input sound pressure. However, it is difficult for the low-sensitivity microphone to detect a small sound with high quality.
In order to cope with these problems, such a microphone which uses a plurality of acoustic sensors having respective different sensitivities has been studied (for example, refer to U.S. Pat. Nos. 8,223,981 and 8,233,637, U.S. Patent Application Publication 2007/0047746 (published on Mar. 1, 2007), and Japanese Unexamined Patent Publication No. 2008-245267 (published on Oct. 9, 2008)).
Each of U.S. Pat. Nos. 8,223,981 and 8,233,637 discloses a microphone including a plurality of acoustic sensors, wherein the plurality of acoustic sensors output a plurality of signals and the plurality of signals are switched or combined in response to a sound pressure. In particular, U.S. Pat. No. 8,223,981 discloses a microphone including a high-sensitivity acoustic sensor whose detectable sound pressure level (SPL) ranges from 20 dB to 110 dB and a low-sensitivity acoustic sensor whose detectable sound pressure level ranges from 50 dB to 140 dB, wherein the microphone uses the high-sensitivity acoustic sensor and the low-sensitivity acoustic sensor in a switching manner so as to achieve a detectable sound pressure level ranging from 20 dB to 140 dB. Moreover, each of Japanese Unexamined Patent Publication No. 2008-245267 and U.S. Patent Application Publication No. 2007/0047746 discloses a configuration including a plurality of acoustic sensors independently provided on a single chip.
However, according to the above configuration described in each of Japanese Unexamined Patent Publication No. 2008-245267 and U.S. Patent Application Publication No. 2007/0047746, the acoustic sensors are formed independently of one another, and therefore there occur a variation and mismatching in their acoustic characteristics. Here, the expression “variation in the acoustic characteristics” refers to a difference between the chips with regard to the acoustic characteristics of the acoustic sensor. The expression “mismatching in the acoustic characteristics” refers to a difference between the plurality of acoustic sensors in a single chip with regard to the acoustic characteristics.
Specifically, the acoustic sensors have thin films warped in respective different manners, so that variations in the detection sensitivity occur between the chips independently. As a result, there occurs a large variation between the chips in the difference between the in detection sensitivities among the acoustic sensors. Further, the acoustic sensors have their respective back chambers and vent holes. Since the acoustic characteristics such as frequency characteristics and phases are affected by the back chamber and the vent hole, mismatching in the acoustic characteristics occurs in the chip.